Satellite communication systems convey information, between satellites in earth orbit and terrestrially-based transceiver stations, by propagating electromagnetic energy across a wireless radio link. Important components in any radio link communications systems are antennas. An antenna is a component that converts an electromagnetic wave propagating on a transmission line to an electromagnetic wave propagating in free space (transmission), or an electromagnetic wave propagating in free space to an electromagnetic wave propagating on a transmission line (reception).
Polarization is an important characteristic of propagating electromagnetic waves. Polarization describes the motion through which an electric field vector of an electromagnetic wave points as the electromagnetic wave travels through a point in space. The electric field vector tip can trace a line, circle, or ellipse as the electromagnetic wave passes through the imaginary point in space. In general, these traces are referred to as linear, circular, or elliptical polarization, respectively.
Polarization is important in many applications, and particularly for antennas. The polarization of the antenna is defined by the field orientation of the electromagnetic wave it radiates when the antenna is transmitting. The polarization characteristic of the antenna is important because, for maximum power transfer between radio links, the transmitting and receiving antennas must be of identical matching polarization states at the same time. If the transmitting and receiving antennas are linear polarized, for example, and if the transmitting antenna is horizontally polarized while the receiving antenna is vertically polarized, then no power would be received. Conversely, if both the transmitting and receiving antennas are horizontally or vertically polarized, then maximum power is received. Because antennas facilitate the transition of electromagnetic energy propagating between free space and a transmission link, polarization is also an important characteristic of all antennas.
A particular satellite may be configured to receive and transmit electromagnetic energy having a specific polarization, e.g., either linear polarization (LP), or circular polarization (CP). Because a terrestrially-based transceiver station may be required to communicate with different satellites, the terrestrially-based transceiver station may need to be able to receive and transmit microwave energy having different polarizations.
An advantage of a radio link using circularly polarized microwave energy is that the angular relationship between the transmitting antenna and the receiving antenna is largely irrelevant. For linearly polarized radio links, however, the angular relationship is important because, as described above, the polarization angle of the transmitter must match the polarization angle of the receiver for maximum power transfer.
For stationary, terrestrially-based transceiver stations, the polarization angle that matches the target satellite may be determined based on the terrestrial location of the transceiver station and the position of the satellite. If the location of the terrestrially-based transceiver station changes, or if the terrestrially-based transceiver stations acquires a different satellite, the polarization angle of the transceiver station may need to change to match the satellite polarization. The ability to change the polarization angle is referred to herein as “polarization skew control.”
Prior art polarization skew control may involve rotating the entire transceiver, which may be heavy and difficult to mechanically move. Further, the rotation may require flexible cables/waveguide assemblies to accommodate the rotation.